A conventional steering wheel for a vehicle has an annular-shaped ring portion and a boss portion disposed at the center of the ring portion. The boss portion connects the steering wheel to the top end of a steering shaft. Spoke portions, radially disposed from the boss portion, connect the ring portion to the boss portion.
This conventional steering wheel has a pad containing functional parts, such as an air bag device, disposed in an upper part of the boss portion. This conventional steering wheel also has a steering wheel body comprising the ring portion, the spoke portions, and the lower part of the boss portion, as described in Japanese Patent Publication No. Hei 6-305427.
The steering wheel body has a core portion and a cover portion. The core portion provides the structural shape of the steering wheel body. The cover portion provides the exterior cover disposed on the core portion. The core portion includes a ring core providing the inner structural element of the ring portion, spoke cores providing the inner structural elements of the spoke portions, and a boss core placed in the lower part of the boss portion. The spoke cores are radially disposed from and are connected at one end to the boss core. The other ends of the spoke cores are connected to the ring core. An upper end of a steering shaft is inserted into the boss core and is retained with a nut, whereby the boss core is connected to the steering shaft.
The cover portion covers the ring core and the spoke cores and extends from the ring core to a position near the pad.
The pad contains the functional parts, such the an air bag device, and is screwed onto the spoke cores and the boss core of the steering wheel body.
The air bag device includes an air bag, folded so that it can be accommodated with the pad, an inflater to expand the air bag with gas, and a bag holder for holding the air bag and the inflater. The air bag device is attached to the steering wheel body through the bag holder.
The air bag has an opening disposed about the inflater that provides a passage to allow the gas from the inflater to flow into the air bag. The inflater includes a main unit having a gas discharge port and a flange formed on the outer peripheral surface of the main unit.
An annular retainer is disposed at the inner circumference of the opening of the air bag. The retainer includes a plurality of bolts projected downwardly that penetrate the peripheral edge of the opening of the air bag. The bolts pass through corresponding holes on the flange of the inflater and can be secured by nuts such that the air bag and the inflater are secured to the bag holder.
However, with the aforementioned conventional steering wheel, the pad, having a separate cover, is placed in the upper part of the boss portion and is attached to the steering wheel body with fixing means. Thus, a large parting line, representing a break in the boundaries between the core portion of the steering wheel body and the cover of the pad, is inevitably formed on the top face of the steering wheel, impairing the design of the steering wheel.
Moreover, accurate assembly is required so that the parting line is uniformly disposed about the periphery of the pad. Thus, strict management of the dimensional accuracy, assembly accuracy, etc., of the parts is required.
Further, to mount the steering wheel on a vehicle, two assembly steps are required. First, the steering shaft is connected to the boss core with a nut before the pad is attached. Then, the pad is attached to the steering wheel body. Therefore, it takes time to mount the steering wheel on the vehicle.
To solve these problems, the cover may be extended to cover the top surface of the steering wheel from the spoke portions to the boss portion. Further, the boss core may be formed with a connection part that allows the upper end of the a steering shaft to be connected from underneath, as described in Japanese Utility Model Publication No. Sho. 63-16267. In other words, if the cover is extended so as to cover the top surface of the steering wheel from the spoke portions to the boss portion, no parting line appears on the top face of the steering wheel. Moreover, if the boss core is formed with a connection part that allows the upper end of the steering shaft to be connected from underneath, attachment of the steering wheel to the steering shaft is accomplished simply on the bottom face of the steering wheel. Thus, the steering wheel can be connected to the steering shaft by executing one step.
However, the structure as described in Japanese Utility Model Publication No. Sho 63-16267 does not have a structure in which functional parts, such as air bag devices, are disposed inside the steering wheel. Thus, functional parts cannot be easily disposed in the boss portion.
Further, such a steering wheel is not provided with a so-called self-aligning mechanism. When an impulsive force acts on a ring portion having the self-aligning mechanism, the plane formed by the ring portion is positioned substantially orthogonal to the action direction of the impulsive force. This positioning distributes the impulsive force across a wide area that can best absorb the impulsive force. To provide the self-aligning mechanism, a plastically deformable part must be disposed between the boss core and a steering shaft, as described in Japanese Patent Publication No. Hei 4-166449.
However, to incorporate the self-aligning mechanism as described in Japanese Patent Publication No. Hei 4-166449 with an air bag device, the number of component parts, the number of assembly steps, and the costs of manufacturing the steering wheel is increased.